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Ceramics are:

inorganic, nonmetallic, solids, crystalline, amorphous (e.g. glass).

Hard, brittle, stable to high temperatures, less dense than metals (up to 40%).

More elastic than metals (do not readily deform under stress).

Very high melting (up to 2800oC).

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Ceramics can be covalent-network and/or ionic bonded.

Typical examples:

Aluminates alumina (Al2O3)Carbides silicon carbide (SiC)Oxides zirconia (ZrO2) and beryllia (BeO)Silicates silica (SiO2)

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Processing of Ceramics

Ceramics are very brittle and shatter when struck.

Bonding prevents atoms from sliding over one another.

Compare steel and a clay pot!

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Processing of Ceramics

Small defects (micro-cracks and voids) developed during processing make ceramics weaker.

Micro-crack areas more susceptible to more stress.

Minimise stress fractures by using very pure uniform particles (< 1m or 10-6m in diameter).

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Processing of Ceramics

Sintering: Heating of very pure uniform particles (about 10-6 m in diameter) under high temp & pressure to force particles to bond.

During sintering the particles coalesce without melting.

Alumina (Al2O3) melts at 2050oC but coalesces at 1650oC

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Processing of Ceramics

Sol-gel process: formation of pure uniform particles.

Metal alkoxide is formed (e.g. Ti(OCH2CH3)4).

Sol formed by reacting metal alkoxides with water (to form Ti(OH)4).

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Processing of Ceramics

Ti(s) + 4CH3CH2OH(ℓ) Ti(OCH2CH3)4(s) + H2(g)

Ti(OCH2CH3)4 + 4H2O(ℓ) Ti(OH)4 + 4CH3CH2OH(ℓ)

Direct addition of water to Ti(s) leads to complex mixtures of oxides and hydroxides.

Alkoxide intermediate ensures a uniform suspension of Ti(OH)4. This is the sol stage.

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Processing of Ceramics

Acidity or basicity of the sol is adjusted to split water from between two of the Ti-OH bonds.

(HO)3Ti-O-H(s) + H-O-Ti(OH)3(s)

(HO)3Ti-O-Ti(OH)3(s) + H2O(l)

This is another example of a condensation reaction.

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Processing of Ceramics

Condensations also occurs at some of the other OH groups.

This produces a three-dimensional network

This is the Gel and the suspension of extremely small particles has the consistency of gelatine.

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Processing of Ceramics

Gel is heated carefully at 200oC to 500oC to remove water and the gel is converted into finely divided oxide powder.

Particle size range of 0.003 to 0.1m in diameter.

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Applications of Ceramics

Used in cutting tool industry (alumina reinforced with silicon carbide).

Used in electronic industry (semiconductor integrated circuits usually made of alumina).

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Piezoelectric materials : Quartz (crystalline SiO2)

Generates an electrical potential after mechanical stress and is used in watches and ultrasonic generators.

Heat resistant tiles: (Silica fibres reinforced with aluminum borosilicate fibres)

Tiles (0.2 g.cm-3) on the space shuttle has surface temperature of 1250oC while shuttle surface temperature gets to 180oC during re-entry.

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Superconductors show no resistance to flow of electricity.

Superconducting behavior only starts below the superconducting transition temperature, Tc.

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Temperature

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Transition temp. (Tc)

Graph of resistance against temperature for a superconducting material

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Meissner effect: permanent magnets levitate over superconductors. The superconductor excludes all magnetic field lines from its volume, so the magnet floats in space.